1. Field of the Invention
The present invention relates to a method of and an apparatus for bonding a light-emitting element in a predetermined position on a board.
2. Description of the Related Art
Generally, a linear array of light-emitting elements such as laser diodes, light-emitting diodes, or the like is used as a light source for image reading and outputting (recording) applications. For example, as shown in FIG. 28 of the accompanying drawings, an LED array 1 comprises a plurality of LED chips (light-emitting elements) 3 mounted on a board 2 at equally spaced intervals and arranged in a linear pattern extending in one direction. The LED chips 3 are bonded on the board 2 by silver paste, with gold wires 4 extending from the respective LED chips 3.
The LED array is required to have the LED chips 3 aligned highly accurately on the board 2 so that the central light-emitting regions of the LED chips 3 will be spaced at equal distances. One known die bonder designed to meet such a requirement is disclosed in Japanese laid-open patent publication No. 6-216170, for example. In the disclosed die bonder, upper and lower solid-state imaging devices are moved to a position between a semiconductor device and a workpiece to which the semiconductor device is to be joined. The upper solid-state imaging device captures the image of a mark on the semiconductor device, whereas the lower solid-state imaging device captures the image of a mark on the workpiece. A processing controller calculates the relative positional relationship between the semiconductor device and the workpiece based on detected signals from the upper and lower solid-state imaging devices. The relative positional relationship between the semiconductor device and the workpiece is adjusted on the basis of the calculated data, and then the semiconductor device is bonded to the workpiece.
On an LED chip, its central light-emitting area and the center of an alignment mark or a contour thereof are usually positionally misaligned with each other. Therefore, even if the LED chips are positioned relatively to the board by aligning the alignment marks of the LED chips with each other, a possible misalignment of the central light-emitting areas of the LED chips cannot effectively be avoided.
When efforts are made to recognize the centers of the contours of the LED chips which are less misaligned with the central light-emitting areas thereof, since the contours of the LED chips tend to vary from chip to chip to a relatively large extent, the centers of the contours of the LED chips are liable to be recognized in error. For this reason, it is difficult to accurately position the central light-emitting regions of the LED chips, resulting in a failure to construct a highly accurate LED array.
Japanese laid-open patent publication No. 7-43112 discloses a method of detecting a light-emitting spot of a light-emitting element and an apparatus for positioning such a light-emitting element. According to the disclosed arrangement, when a semiconductor laser chip is attracted by a suction nozzle and fed to a positioning location, an electric current is supplied to the semiconductor laser chip to enable the semiconductor laser chip to emit light, and a CCD camera positioned in confronting relation to the light-emitting area of the semiconductor laser chip captures an image of the semiconductor laser chip. The captured image is supplied from the CCD camera to a controller, which detects the position and orientation of the semiconductor laser chip based on the supplied image. Based on the detected position and orientation, the controller then controls the suction nozzle to correct the attitude of the semiconductor laser chip.
The principles of the disclosed invention are, however, based on the configurations of semiconductor laser chips, and are not applicable to LED chips whose light-emitting areas are of a comparatively complex shape. In addition, whereas the light emission of a semiconductor laser chip can easily be detected while the semiconductor laser chip is being attracted because the attracted surface of the semiconductor laser chip is different from the light-emitting surface thereof, it would be difficult to detect the center of the light-emitting area of an LED chip as the attracted surface of the LED chip is oriented in the same direction as the light-emitting surface thereof. Furthermore, when a probe would be applied to an LED chip to enable the LED chip to emit light, the probe would shield the light-emitting area of the LED chip, with the result that the center of the light-emitting area would not be detected with high accuracy.
Heretofore, the technique disclosed in Japanese laid-open patent publication No. 6-334022, for example, is known for bonding a plurality of LED chips on a board. According to the disclosed bonding process, an alignment mark on the board and alignment marks on the LED chips are read by individual cameras, and a biaxially movable stage, which supports the board and are movable along X- and Y-axes, is operated on the basis of positional information representing the read alignment marks, after which the LED chips are bonded on the board.
In the disclosed bonding arrangement, the bonding accuracy of the LED chips depends largely on the accuracy with which the biaxially movable stage is positionally measured. The position of the biaxially movable stage is usually measured by encoders or linear scales mounted on slide guides. However, since pitching and yawing displacements of the upper surface of the biaxially movable stage, which serves as a workpiece support, cannot be fully measured, the bonding accuracy of the LED chips tends to be lowered.
According to another bonding process, which is based on the above disclosed bonding process, the LED chips are simultaneously imaged by the camera, and then positionally corrected in order to equalize the distances between the alignment marks on the LED chips before the LED chips are bonded on the board.
However, the other bonding process is disadvantageous in that if the LED chips are widely spaced apart, then they cannot be simultaneously recognized. Another problem is that since the LED chips themselves are tiny pieces, it is difficult to apply alignment marks to the LED chips. In addition, the bonding process is not versatile as it can be applied to transparent boards only.